1. Technical Field of the Invention
The present invention relates to optical transmission lines and optical transmission systems for reducing timing jitter by suppressing optical pulse waveform degradation due to nonlinearity, dispersion and higher-order dispersion of optical fibers in ultra-high-speed optical transmissions.
2. Background Art
In ultra-high-speed optical transmissions which require short optical pulses of a few picoseconds or less, nonlinearity, dispersion and higher-order dispersion in the optical fibers causes the optical pulse waveforms to markedly degrade. Pulse waveform degradation which is due to the effects of higher-order dispersion and nonlinearity occurs because a large portion of the optical spectrum shifts to lower frequencies when the dispersion in the optical fiber is zero or extremely small. This type of pulse waveform degradation forces the transmission distance to be restricted. As a conventional method of compensating for waveform degradation due to nonlinearity and dispersion, there is a method wherein long-distance transmissions are performed by using a special type of optical pulse, called an optical soliton, which can propagate through an optical fiber while maintaining the shape of the waveform by balancing out the dispersion and nonlinearity.
When actually forming an optical soliton transmission system, amplified spontaneous emission noise (ASE) emitted from optical amplifiers used to compensate for loss in the optical fibers causes random changes in the carrier frequencies of the optical solitons. These changes are referred to as the Gordon-Haus effect (see Reference 1: A. Hasegawa et al., "Solitons in Optical Communications", Oxford Univ. Press, 1995), and are known to cause fluctuations in the propagation time of each optical soliton in the optical fiber, thus causing timing jitter. Additionally, the carrier linewidth of the optical soliton source also causes timing jitter in a similar process (see Reference 2: K. Iwatsuki et al., IEEE J. Lightwave Technol., 13, pp. 639-649, 1995).
Therefore, since the timing jitter in optical soliton transmissions restricts the transmission speed or transmission distance, methods have been proposed for reducing the timing jitter by using optical filters having central frequencies which are slid in correspondence with the transmission distance, optical filters and intensity modulators, and the like, and optical soliton transmissions at transmission speeds of approximately 20 Gb/s have been confirmed by recirculating loop experiments (see Reference 1). However, in the above-described conventional methods, the bandwidths of the optical filters must be about 4-5 times those of the signal spectra in order to preserve the waveforms, and optical filters with narrower bandwidths cannot be used.
Since timing jitter accumulation increases with increases in the transmission speed, more effective reduction of timing jitter is a significant step in realizing ultra-high-speed optical soliton transmissions exceeding a few tens of Gb/s. Additionally, in optical soliton transmissions, the spacing between the positions of the optical amplifier and the above-mentioned optical filter, intensity modulators and the like must be made sufficiently short with respect to the soliton period defined by the optical soliton pulsewidth and the mean dispersion value of the optical fiber, so that if the soliton period is shortened for increased transmission speed, the amplifier spacing cannot be maintained at a practical length.
A method of using a dispersion decreasing fiber (DDF) wherein the dispersion value decreases like the attenuation of the optical power for the transmission path has been proposed as a solution to the above-mentioned problem of the amplifier spacing (see Reference 3: A. J. Stentz et al., Opt. Lett., 20, pp. 1770-1772, 1995). In a series of DDFS, the amplifier spacing can be maintained by designing the length and dispersion distribution of the DDF such that optical solitons having periods of a few picoseconds or less can be made to propagate while retaining their waveforms due to a localized balancing of the nonlinearity and dispersion.
On the other hand, when performing long-distance transmissions using conventional DDFS as described above, the dispersion value of the DDF near the output end becomes small if the mean dispersion value is reduced to account for the accumulation of timing jitter, as a result of which the pulse waveform degrades under the influence of higher-order dispersion and nonlinearity (see Reference 4: K. Suzuki et al., OAA' 95 FB3, 1995) so as to restrict the transmission distance. As mentioned above, when the dispersion of the optical fiber is either zero or extremely small, the degradation of the pulse waveform due to higher-order dispersion and nonlinearity occurs because a large portion of the optical spectrum shifts to lower frequencies. No specific solutions to this type of waveform degradation have heretofore been proposed. Additionally, while timing jitter also occurs in methods using DDFs as transmission lines due to the application of optical soliton effects, no methods have been conventionally proposed for effectively reducing timing jitter in this case as well.